As we know that nephrotic syndrome is characteristically present with proteinuria and on the other hand haematuria is seen in nephritic syndrome. Defect/loss of podocytes in nephrotic syndrome leads to escape of proteins , so why don't RBC's escape with the proteins?

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If you remember the structure of the glomerulus there are three barriers that regulate the filtration process: the first barrier is endothelial cells which has spaces between them that allow the passage of molecules of certain size, and among the molecules these spaces allow to pass are proteins (Ig, albumin,different transportation proteins, etc...) but such spaces are so small (70-80 nm at maximum) and do not allow red cells to pass (diameter between 6-8 micrometeres which is much bigger than 80 nanometers). (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2681366/)

The second barrier is the basement membrane which also do not allow large structures like different cells in the blood to pass but allow small molecules to pass. Remember that glomerular filtration is not governed only by size of molecules, but the charge of the molecules play a role, for example the basement membrane is negatively charged which leads to repulsion of molecules with negative charges like albumin and do not allow them to pass. If i am not mistaken the podocytes also contribute to the regulation of charge in basement membrane and problems in podocytes could lead to alteration in charge in basement membrane which could contribute to albumin passage into tubules. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2895306/)

And the third barrier is the podocytes which produce the basement membrane among the other functions that they have. The podocytes also regulate filtration by contracting and restricting spaces between their foot processes which leads to decrease in filtration. (https://academic.oup.com/jes/article/4/4/bvaa029/5781251)

Podocytes allow micromolecules to pass like electrolytes and glucose, drugs, etc... through the spaces between their foot process (called slit pore) but do not allow macromolecules to pass like albumin (although a very small quantity can pass but majority do not). (https://www.hindawi.com/journals/ijn/2012/481520/)

As you can see even if there is an abnormality in podocytes at maximum it will allow macromolecules to pass like albumin, because larger structures like red cells will be blocked by endothelial cells and basement membrane before they even reach the podocytes level.


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